"Cell Population Kinetics as Factor in Tumor Therapy" aims at a quantitative analysis of the effects of therapeutic perturbations on the kinetics of tumor growth. Our chosen assay procedure, the 125I-iododeoxyridine (125IUdR) prelabeling technique, is now sufficiently refined to permit detailed observations on the rate of tumor cell proliferation, migration, and death of intraperitoneally growing L1210 cells. Preliminary studies indicate that the 125I assay can also be applied to extraperitonally growing tumor cells. Ideally, our assay should thus permit continuous evaluation of the success of various radiotherapeutic or chemothrapeutic treatment regimens on the rate of death, migration, and proliferation of tumor cells growing in different body compartments. More specifically, the effects of radiation and antimetabolites on tumor growth parameters, as influenced by changes in nutrition, oxygenation, repair capacity, cell concentration, and other variable factors will be studied throughout tumor growth. Attempts will be made to manipulate these factors in such a way that the treatment damage to the tumor cells is maximized while the damage to the host tissues is kept as low as possible. At present, the most promising approach appears to be combined administration of hyperthermia and chemical radiosensitizers (Nitroimidazoles) which yields radiation dose modifying factors up to 4.2, as compared to 3.1 for full oxygenation. BIBLIOGRAPHIC REFERENCES: Hofer, K.G., Choppin, D.A. and Hofer, M.G., Effect of hyperthermia on the radiosensitivity of normal and malignant cells in mice. Cancer (in press), 1976. Warters, R.L., Hofer, K.G., Harris, C.R. and Smith, J.M., Radionuclide toxicity in cultured mammalian cells: Elucidation of the primary site of radiation damage. Current Topics in Radiat. Res. Quarterly (in press), 1976.